1. Field of the Invention
The present invention relates to a damping force variable shock absorber having a damping force variable valve assembly, and more particularly, to a damping force variable shock absorber in which an additional valve is installed in a rod guide so as to implement performance that could not be implemented by a main valve.
2. Description of the Related Art
In general, a shock absorber is installed in means of transportation such as automobile or the like, and improves a ride comfort by absorbing and damping a vibration or shock from a road surface during driving.
Such a shock absorber includes a cylinder and a piston rod installed to be compressible and extendable within the cylinder. The cylinder and the piston rod are installed in a vehicle body, wheels, or axles.
A shock absorber, a damping force of which is set to be low, can improve a ride comfort by absorbing a vibration caused by unevenness of a rod surface during driving. On the contrary, a shock absorber, a damping force of which is set to be high, can improve a steering stability by suppressing a change in a posture of a vehicle body. Therefore, in a conventional vehicle, a shock absorber, to which a damping force characteristic is differently set, is applied according to the purpose of use of the vehicle.
Recently, there has been developed a damping force variable shock absorber in which a damping force variable valve capable of appropriately adjusting a damping force characteristic is mounted on one side of a shock absorber, such that a damping force characteristic is appropriately adjusted to improve a damping force a ride comfort or a steering ability according to a road surface and a driving condition.
FIG. 1 is a sectional view illustrating an example of a conventional damping force variable shock absorber. Referring to FIG. 1, the conventional damping force variable shock absorber 10 includes a base shell 12, and an inner tube 14 which is installed inside the base shell 12 and in which a piston rod 24 is installed to be movable in a length direction. A rod guide 26 and a body valve 27 are installed in an upper portion and a lower portion of the inner tube 14 and the base shell 12, respectively. In the inside of the inner tube 14, a piston valve 25 is connected to one end of the piston rod 24, and the piston valve 25 partitions the internal space of the inner tube 14 into a rebound chamber 20 and a compression chamber 22. A top cap 28 and a base cap 29 are installed in an upper portion and a lower portion of the base shell 12, respectively.
A reservoir chamber 30 is formed between the inner tube 14 and the base shell 12 to compensate for a change in a volume of the inner tube 14 according to a reciprocating motion of the piston rod 24. A flow of a working fluid between the reservoir chamber 30 and the compression chamber 22 is controlled by the body valve 27.
In addition, a separator tube 16 is installed inside the base shell 12. Due to the separator tube 16, the inside of the base shell 12 is partitioned into a high-pressure chamber PH connected to the rebound chamber 20, and a low-pressure chamber PL serving as the reservoir chamber 30.
The high-pressure chamber PH is connected to the rebound chamber 20 through an inner hole 14a of the inner tube 14. Meanwhile, the low-pressure chamber PL is connected to the compression chamber 22 through a lower passage 32 formed between a body of a body valve 27 and the base shell 12 (or the base cap 29) and a passage formed in the body valve 27.
Meanwhile, the conventional shock absorber 10 includes a damping force variable valve assembly 40 mounted on one side of the base shell 12 so as to vary a damping force.
The damping force variable valve assembly 40 is provided with oil passages connected to a base shell 12 and a separator tube 16 and communicating with the high-pressure chamber PH and the low-pressure chamber PL, respectively. In addition, the damping force variable valve assembly 40 includes a spool 44 installed to be moved by a driving of an actuator 42. An inner passage communicating with the high-pressure chamber PH and the low-pressure chamber PL is varied by a movement of the spool 44, and a damping force of the shock absorber 10 is varied accordingly.
However, the conventional damping force variable shock absorber could not perform a frequency selective function or an amplitude selective function that adjusts a damping force according to a frequency or amplitude of an input shock, and could not generate a damping force when a working fluid inside the shock absorber flows at an extremely low speed. Therefore, in designing a damping force variable shock absorber, there is a need to continuously conduct research and development on methods capable of selectively mounting an amplitude selective valve, a frequency selective valve, and an extremely-low-speed valve on a damping force variable shock absorber according to a vehicle type to be applied.